EMD-type unit injectors are injectors of a type manufactured or formerly manufactured by EMD. They feature mechanical control of timing, as distinguished from electronic (solenoid) control, and are well known to those familiar with the art to which the invention relates. EMD-type injectors are widely used to power railroad engine locomotives, and continue to compete successfully with electronically-controlled designs.
One aspect of these systems is injection timing. The current method for adjusting injection timing on EMD engines is to set the injector's port-closing position with reference to the engine cam position. With roots-blown engines this is done when the cylinder piston is two degrees before top dead center. For turbo-charged engines this is done when the cylinder piston is in its top dead center position. In both instances, the associated engine cam follower is still in following contact with the base circle of the associated engine cam, that is, there is maximum spring-driven retraction of the drive linkage that powers the injector pump plunger, but this retraction may vary somewhat from a desired norm due to variations in conditions encountered when installing or reinstalling the injector, or due to wear of elements of the drive linkage during use, thereby causing improper timing.
The drive linkage that powers the injector pump includes a rocker arm assembly that actuates the injector plunger as determined by the engine cam profile. Adjustment of injection timing is done by turning an adjusting screw on the output end of the engine rocker arm. The drive linkage that powers the plunger includes (a) the associated engine cam, (b) a rocker arm assembly, including a rocker arm proper, a cam follower at the input end of the arm and the said adjusting screw at the output end, (c) a "button" or socket pad on the head of the adjusting screw and forming, together with the head, a ball-and-socket joint, and (d) a spring-loaded tappet or follower carried by the injector body and slidably engaged by the pad in a manner to accommodate the slight variance between the rocking motion of the adjusting screw and pad and the strictly rectilinear motion of the tappet. The tappet and plunger are in end-to-end engagement and are linked together for movement together in both directions.
Turning the adjusting screw changes the free length of the adjusting screw below the output end of the rocker arm proper, and has the effect of shortening or lengthening the drive linkage, which may need adjustment upon installation of the injector, or may have experienced significant wear during service. The linkage length is changed in this manner until there is a certain specified timing distance between the top face of the tappet and a fixed surface, namely the top flat face of the injector body. Such specified timing distance is the distance that obtains when the cut-off helix of the plunger is at some predetermined remove above the point at which it will close off its associated spill port in the plunger bushing to thereby initiate injection. When this specified timing distance obtains, the drive linkage is in properly adjusted position, and the adjusting screw is at what may be referred to as its set point. Such specified timing distance is usually listed on the engine manufacturer's data plate. Setting gages are provided so that the specified timing distance may be more readily confirmed, and more readily adjusted to if not initially confirmed. Once the adjusting screw is confirmed to be at its set point, it is fixed there by tightening an associated lock nut.
All this is well known to industry mechanics, who often carry in memory such specified timing distance for a particular engine model or models, or perhaps different specified timing distances for different engine models.
There is continuing pressure from regulatory agencies and public opinion to reduce nitrous oxide emissions from locomotive engines and other diesel engines. As a result, the railroad, diesel engine, and fuel injection equipment industries are continually reviewing the design and performance characteristics of the various components of the several interrelating systems that contribute to engine emissions and other performance characteristics.
Injection timing is an important component of these systems. Changes in timing have significant effects on fuel economy, engine noise, and emissions. Retarding injection timing is presently one of the readily available methods, combined with one or more others, to reduce nitrous oxide emissions. A usual method of retarding injection is by simply changing or re-specifying the specified timing distance, and then rotating the timing screw until the new specified timing distance obtains.
However, adjustment of injection timing from its originally intended setting by rotation of the adjusting screw has several disadvantages. Doing so amounts to establishing a new screw set point, and correspondingly, a new specification for the desired distance from the top of the follower to the top face of the injector body, inconsistent with the timing distance specified on the engine data plate. A new setting gage and new engine marking must be provided.
Moreover, the new timing distance specification or adjusting screw set point prescribed to retard injection timing and thereby meet emission requirements may be different for different engine models, even though the old timing distance specification or adjusting screw set point for those models was the same, compounding the opportunity for error by a mechanic in setting the correct new injector timing.
When changing injection timing or making any other modifications or adjustments in the equipment, a number of factors must be taken into consideration and balanced against each other. One factor is engine fuel efficiency. The effect on engine fuel efficiency of any change in injection timing is an important consideration. From a fuel efficiency standpoint, a method of retarding injection timing that is accompanied by better fuel efficiency than that resulting when the retarding method is simply rotating the adjusting screw to a new setting, corresponding to a new specified timing distance, is obviously to be preferred.